Apparatus for the current limiting interruption of currents at high voltages

ABSTRACT

An improved circuit for the current limiting interruption of currents in a power main at high voltages of the type wherein an energy absorbing circuit and a capacitor are each connected across the interrupter or commutation switch, and the capacitor is precharged so that upon opening of the switch the discharge current of the capacitor will oppose the current flowing through the switch and cause it to flow through the energy absorbing circuit. Circuit means are provided which causes charging of the capacitor only during the opening of the interrupter switch to a value which is dependent on the arc voltage at the interrupter switch and/or the current in the main so that sufficient time is available to the switch during the commutation phase to increase its arc resistance to an extent sufficient to commutate the main current into the parallelly connected capacitor and accordingly into the energy absorber and prevent the main current from commutating back into the switch.

United States Patent 1191 Kind et a];

[ 41 APPARATUS FOR THE CURRENT LIMITING INTERRUPIION or CURRENT AT HIGHVOLTAGES Dieter Kind, Pockelstrasse 4; Hagen Hal-tel, both ofBraunschweig; Jurgen Salge, Salzgitter-Osterlinde; Rudolf Brilkn,Salzgitter-Lebenstedt, all of Germany 111 Kind, by m1 Bartel, Salge,and- Brilka I Inventors:

[73] Assignee:

Filed: Aug. 4, 1971 Appl. No: 168,967

Foreign Application Priority Data Aug. 4, 1970 Gennany P 20 38 624.7

Us. or 317/11 A, 317/11 B, 317/11 E,

' 307/136 int. 131. 110211 7/22 Field of Search 317/11 A, 11 1a, 11 D,

317/11 E, 11 R; 307/136; 200/144 AP Hsi [111 3,753,042 1451 Aug. 14,1973

Primary Examiner-D. F. Duggan Assistant Examinerl-larvey FendelmanAttorney-George H. Spencer et al.

[57] ABSTRACT An improved circuit for the current limiting interruptionof currents in a power main at high voltages of the type wherein anenergy absorbing circuit and a capacitor are each connected across theinterrupter or commutation switch, and the capacitor is precharged sothat upon opening of the switch the discharge current of the capacitorwill oppose the current flowing through the switch and cause it to flowthrough the energy absorbing circuit. Circuit means are provided whichcauses charging of the capacitor only during the opening of theinterrupter switch to a value which is dependent'on the arc voltage atthe interrupter switch and/or the current in the main so that sufficienttime is available to the switch during the commutation phase to increaseits arc resistance to an extent sufficient to commutate the main currentinto the paralleiiy connected capacitor and accordingly into the energyabsorber and prevent the main current from commutating back into theswitch.

10 Claims, 8 Drawing Figures p Ampl/f/er R I (v z A A l S J (R;Rui-om/fi unum 4151a 3,

sum 2 or 2 WT FIG 5 a D I L FQEG) 1? Hi3}: (I; Q? V 5 (IQ/52501? L];

F52 6 3 nc BACKGROUND OF THE INVENTION The invention relates to animproved apparatus for the current limiting interruption of direct oralternating currents at high voltages. More particularly the presentinvention relates to an improved apparatus for this purpose in which acapacitor is connected across a switch (commutation switch) used toswitch off the current so that the discharge current from the capacitoropposes the current to be switched off; this discharge current being ofsuch magnitude that the current is forced to commutate to a parallelpath having a high resistance so that it is reduced to a quiescentcurrent value which is switched off by a subsequently connected circuitbreaker.

With a current-limiting switch-off in direct current as well asalternating current mains, the switching device must produce a switchvoltage after actuation which is greater than the driving voltage in thecircuit and it must absorb the energy furnished by the mains (includingthe energy stored in the inductances of the mains) during the switchingprocess. These two requirements can be met up to certain current limitsby switches having intensive cooling of the arc.

As is known a switch can also be greatly relieved if bypasses are usedfor the voltage generation and for the energy conversion. This is done,for example, by connecting a capacitor, which is either precharged oruncharged, and an energy absorber in parallel with the commutationswitch. The current in the switch is sufficiently reduced aftercommutation to the energy absorber so that it can be switched off by aquiescent current switch. With suitable dimensioning of the main switch,it is possible to precharge the capacitor at a certain level for a fixedcurrent so that the current will be more easily commutated into theauxiliary commutation circuit and subsequently into the absorber circuitthan would be possible if the capacitor were not precharged. Thedrawback of this process is that precharging of the capacitor requiresadditional energy and the circuit is optimally designed only for onecurrent value. A current interrupting arrangement such as illustrated inH6. 1 was for example proposed by D. Kind, E. Marx, K. Mollenhoff, J.Salge in the paperzCircuit- Breaker for HVDC-transmissions. CIGRE 1968,Report No. 13-08.

This method for the current limiting interruption of a current at a highvoltage is shown schematically in FIG. 1, wherein L indicates theinductance of the mains or conductor, S is a commutation switch, forinstance a circuit-breaker with intensive cooling of the arc and S is aquiescent current switch. An auxiliary commutation circuit with anauxiliary switch HS, a capacitor C and a leakage inductance L,, and anabsorber circuit A containing an energy absorbing element such as anohmic resistor R, a voltage dependent resistor R(i) or an inductance L,are each connected in parallel with the commutation switch S. The mainsvoltage V causes a current i to flow through this circuit which currentinitially flows only as the switch current 1', through the commutationswitch S and quiescent current switch S The capacitor C is assumed to beprecharged with the indicated polarity by an external current source.When switch S is opened in order to interrupt the flow of current in themains, the arc voltage produced will eventually cause switch- HS toclose, resulting in the circuit formed by C and L, being connected inparallel with switch S. As a result, a capacitor current i flows fromcapacitor C via the leakage inductance L,, toward switch S in adirection opposed to the direction of flow and current L, so thatcurrent i, is reduced to a greater or lesser degree depending on itsmagnitude and thus effects commutation of the current i to the absorbercircuit. The current i is reduced by the resistances in this absorbercircuit A and is then finally interrupted by the. quiescent currentswitch or circuit breaker S,;.

This commutation of the mains current i to the absorber circuit issimpler the more time is available to switch S for this process. FIG. 2shows the time sequence of the switch current i. for'two differentvalues of mains currents l and I: with different capacitor precharges O.For sequences 1 and 3 the capacitor C is precharged with the same chargeQ1, While for sequence 2 the capacitor C is precharged with a change Qwhich is less than Q For the curves 1 and 2 the precharges are soselected that the zero line is just being touched at a minimum of thecurrent curves. When the current approaches zero in this manner, switch.S has more time for the commutation of the current to the parallelpathsand the possibility of a successful commutation is thus much greaterthanin the case of curve 3 where the current i, intersects the zero linewith a large slope. However, the current path i, which is favorable forthe commutation, as it is shown for example in.

curves 1 or 2 of FIG. 2, can be obtained only for a certain currentvalue when the capacitor precharge is constant.

, SUMMARY OF THE INVENTION The above drawbacks of the prior artarrangements of this type are overcome according to the invention byproviding an additional means for causing the capacitor to be prechargedto a value dependent on the current to be interrupted and/or the arcvoltage and for effecting the current discharge path so that the switchhas sufficient time during the commutation phase to increase its arcresistance to such an extent that the current is commutated into theparallel paths containing the capacitor and the energy absorber,respectively, and to prevent the current from commutating back into theswitch.

A number of embodiments of circuit arrangements according to theinvention are presented. According to one embodiment of the inventionthe capacitor is precharged by the output current of an amplifier whichoutput current is proportional to the current through the interrupterswitch and the arc voltage across the switch.

According to a further feature of the invention an additional inductanceor resistance is connected in series with the capacitor, or in serieswith commutation switches to increase the commutation time of theinterrupter switch.

According to a further embodiment of the invention the capacitor ischarged in the proper polarity via resistors connected to opposite sidesof the interrupter switch and the discharge circuit iscompleted by arevoltage responsive switches connected in series with the capacitor oneither side thereof. Additionally, a plurality of such capacitors andare voltage responsive switch combinations may be connected in series soas to provide a multiplication of the charging voltage.

7 According to still a further embodiment of the invention a furtherinterrupter switch is connected in series with the first interrupterswitch, and the series connection of a further capacitor, which has alarge capacitance and a low rated voltage relative to the firstcapacitor, and a switch responsive to the arc voltage of the furtherinterrupter switch is connected in parallel with the further interrupterswitch.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagramillustrating the principle of a circuit arrangement with a commutationdevice according to the prior art.

'FIG. 2 is a current time diagram used in explaining the principle ofthe invention.

FIG. 3 is a schematic'circuit arrangement according to the inventionutilizing current and voltage depen- DESCRIPTION OF THE PREFERREDEMBODIMENTS Referring now to FIG. 3 there is shown a first embodiment ofan improved commutation circuit according to the invention so as toprovide current dependent charging of the commutation circuit capacitorC. In this and all succeeding Figures the same reference characters usedin FIG. 1 are utilized to designate similar components. According tothis embodiment of the invention the energy for charging capacitor C isprovided by the switching are created when switch S is opened onlyduring the switching process and is dependent on the arc voltage V andon the switching current i,. In order to provide this charging currentfor the capacitor C, a measuring resistor Rm is connected in series withswitch S and the voltage thereacross and the voltage across the switchS, i.e., the arc voltage V,,, are combined in a precharger P, i.e., anamplifier, which produces a charging current i f (V,,, i,). Moreover,

diagram an additional inductance I... which may also becurrentdependent, i.e. L, (i), or an ohmic resistance R, is connected inthe commutation circuit in series with the leakage inductance L,. Theseelements serve to reduce the frequency of the oscillating or dischargecurrent so that in the vicinity of the point when current 1, becomeszero, its curve is sufficiently flat that the switch has sufficient timefor commutation. After the subsequent closing of switch HS as a resultof the arc voltage V reaching a certain value, the capacitor C, whichhas been charged with the illustrated polarity by means of the output ofamplifier P, begins to discharge and sends the discharge current I,through switch S in a direction opposite that of current i. Thecapacitor precharge and the switching elements C, L,, L: or Rrespectively, are so dimensioned that the switching current i, in thevicinity of its becoming zero follows a flat path, as shown in curve 1or 2 of FIG. 2, and current i commutates into the energy absorbercircuit A connected in parallel with the switch S. The amplifier P is anamplifier with electron circuit packages and the supply voltage of theamplifier P is the arc voltage V of the switch S. The value of thecharge current i, is varied by the voltage i, R which is applied to theinput of amplifier P.

Capacitor C can also be charged by means of a switching arrangement asshown in FIG. 4a.v Here the arc voltage V appearing across switch S isapplied to capacitor C in the proper polarity via a pair of chargingresistors R Additionally, in place of the single auxiliary switch HS, inorder to switch on the commutation circuit, a pair of spark gap typeswitches F S, of the type well known in the art, are connected in serieswith the capacitor C, one on either side thereof. The effect of thiscircuit is such that when switch S is opened, an arc voltage V, isproduced which voltage is produced in this case at a switch havingarc-cooling.

By suitably selecting the charging time constant 1 2R C, it is possibleto precharge capacitor C to a somewhat smaller value for a small currentI, until the actuation voltage V, of the spark gap paths FS at time t I,is reached than for a larger current I, as can be seen in FIG. 4b forcurves 1 and 2.

For switches with a very low arc voltage or so as to enable betterutilization of the voltage capacity of the commutation capacitance, thecircuit of FIG. 4 may be "modified by using a voltage doubling circuitas shown in FIG. 5. In this embodiment the total capacitance of thedischarge capacitor C of FIG. 4 is provided by two capacitors, eachhaving a capacitance value of 2C, which are connected in series via anadditional spark gap F 8,. The individual commutation capacitors 2C arecharged by the arc voltage V, in parallel via resistors R R during thebeginning of the switching or commutation process and are discharged inseries via switch S when the spark gaps or paths FS FS are triggered asa result of the arc voltage reaching the value Va. In this waycapacitors can be precharged so that they provide four times the energy,provided in the simple precharging circuit according to FIG. 4a, withthe other parameters of the circuit remaining the same. An n-foldmultiplication of the charging voltage is possible with the circuitaccording to FIG. 6 which is similar to FIG. 5 but contains 11capacitors each having a value of nC connected in series via spark gapsF S and charged in parallel via resistors R R by the arc voltage V,,. Inorder for all n charging currents i to have the same time constant: v TCOnSt n C (Rum- RLM); I: I, 2, 3, n -8- "'s n C u u) Furthermore aftercommutation of the current i to the absorber circuit A (R(i) or L inorder for the partial capacitors n C to be charged with identicalvoltages (V const), the charging resistances R must be dimensioned asfollows:

m RLI" U )/2]/n wherej== I, 3,5,. (Zn-l) RLK= Riot/2v" wherek=2, 4, 6,2n

I if all of the resistances R with m l, 2, 3, 2n consist of the seriesconnection of the basic unit R jn, the voltage load and the energy losswith respect to the unit of length are identical for all resistors.The'charging resistance R and thus r must also be so dimensioned thatthe capacitors nC are sufficiently charged in the few millisecondsbefore commutation or switching of the current i occurs, but that aftercommutation the parallel circuit containing the capacitors and chargingresistances is highly resistant with respect to the resistance of theabsorber circuit A. When suitable spark gaps or paths F8, are used, e.g.spark gaps capable of carrying a current of several hundred to onethousand amperes for several ms, the charging resistors R maysimultaneously be dimensioned to serve as energy absorbers.

A further circuit arrangement for the current dependent precharging of acommutation capacitance is shown in FIG. 7. in this embodiment a furthercommutation or interrupter switch S, is connected in series with thecommutation switch S, (which corresponds to switch S of FIG. 1) and aseries circuit including a capacitor C, which has a large capacitancebut a low rated voltage, an auxiliary switch HS,, and a leakageinductance L is connected in parallel with the commutation switch S,.Capacitor C, is charged by the mains current and is subsequentlydischarged to the capacitor C, which has a low capacitance but highrated voltage. Both of the switches S, and S, are triggered at the sametime by an opening command. In the operation of this circuit when switchS, is opened, it produces an arc voltage. The capacitance of thecapacitor C, is so great that after closing of auxiliary switch HS, bythe arc voltage across 8,, commutation of the current i to capacitor C,is assured and capacitor C, is now charged by the main current i If theauxiliary switch HS, is a spark gap or path, the charging occursindependently of the magnitude of the main currents i up to a fixedvoltage. When switch HS, is an auxiliary switch which is switched onafter a constant time interval At upon actuation of switch HS,, theprecharging of capacitor C, is proportional to the magnitude of thecurrent i due to the relationship At const V,;, (i/C,) 'At).

The circuit is so dimensioned that capacitor C, can be charged only to afew kV and then discharges, upon closing of H8 in the given directionvia,circuit C L 8,, with the discharge current i, producing a zero current passage in switch 8,, thus causing a commutation of the mainscurrent to the absorber circuit A (R, R(i) or L). After quenching of thearc of switch S, and closing of HS: the circuit exhibits the approximatebehavior (due to C, C as if the precharged capacitor C (same charge asin C,) were discharged across 8;.

The disclosed circuit arrangements can also be provided at alternateends with means which effect an additional influence on the current pathif these means are not provided in the respective circuit arrangementper se.

It will be understood that the above description of the presentinvention is susceptible to various modification. changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents oi the appended claims.

We claim:

1. In a circuit arrangement for the current limiting interruption ofdirect and alternating currents at high voltages flowing in a power mainhaving a commutation switch connected in said main, an energy absorbinghigh resistance circuit, and a series circuit including a capacitor, aleakage inductance and a normally open auxiliary switch means responsiveto the arc voltage of said commutation switch, said energy absorbingcircuit and said series circuit each being connected in parallel withsaid commutation switch, said capacitor being precharged so that uponopening of said commutation switch the discharge current from saidcapacitor will flow into said commutation switch in a direction oppositethat of the current to be interrupted, said discharge current being ofsuch magnitude that the current to be interrupted will commutate to atleast said energy absorbing high resistance circuit wherein the currentis reduced to a quiescent current value which is switched off by acircuit breaker subsequently connected in said main, the improvementwherein said circuit arrangement further includes circuit means forinfluencing the precharging of said capacitor and the capacitordischarge current path so that sufficient time is available to saidcommutation switch during the commutation phase to increase its arcresistance to such an extent that the current flow therein which is tobe interrupted is commutated into the parallelly connected circuits andcannot commutate back into said commutation switch; and wherein saidcircuit means includes an additional inductance connected in series withsaid capacitor, said additional inductance being designed so that itdelays the commutation process to the extent required for an increase inthe arc resistance of said commutation switch, and means for causingsaid capacitor to be charged only during the commutation process.

2. The circuit arrangement defined in claim 1 wherein said circuit meansincludes means for charging said capacitor by the arc voltage.

3. The circuit arrangement defined in claim 2 wherein said means forcharging causes said capacitor to be charged to a value dependent on theintensity of the current to be'interrupted.

4. The circuit arrangement defined in claim 3 wherein said means forcharging said capacitor comprises: a measuring resistor connected inseries with said commutation switch and an amplifier circuit having itssignal input connected across said measuring resistor and its supplyvoltage input connected across said commutation switch said amplifierprovided the output value i, =f(i, V,,) where i, is the charging currentfor said capacitor, i is the current to be interrupted and V, is the arcvoltage across said commutation switch.

5. The circuit arrangement defined in claim 3 wherein said auxiliaryswitch means comprises a pair of spark gaps, one of which is connectedon each side of said capacitor, in said series circuit, and wherein saidcharging means comprises a pair of resistive circuit paths connectingthe terminals of said capacitor across said commutation switch.

6. The circuit arrangement defined in claim 5 wherein said capacitorincludes a plurality of capacitors connected in series; wherein saidpair of resistive circuit paths connects the terminals of each of saidcapacitors in parallel across said commutation switch so as to chargeeach capacitor in the same polarity; and wherein an additional spark gapis connected in said series circuit between each pair of capacitors,whereby a multiplier type charging circuit is provided.

7. The circuit arrangement defined in claim 6 wherein there are ncapacitors and wherein all n chargmutation of the current to beinterrupted into the parallely connected high resistance circuit branchwith identical voltages.

8. The circuit arrangement defined in claim 6 wherein the charge timeconstants of said charging means are dimensioned so that the capacitorsare sufficiently charged in a few milliseconds before the commutation ofthe current to be interrupted occurs and during the duration of the arcformed by the opening of said commutation switch.

9. The circuit arrangement defined in claim 6 wherein said spark gapsare of sufficient size so that they can carry a current of from severalhundred to one thousand amperes for several tens of milliseconds, andwherein the charging resistances in said resistive paths are dimensionedso that they simultaneously serve as energy absorbers.

10. In a circuit arrangement for the current limiting interruption ofdirect and alternating currents at high voltages flowing in a power mainhaving a commutation switch connected in said main, an energy absorbinghigh resistance circuit, and a series circuit including a capacitor, aleakage inductance and a-normally open auxiliary switch means responsiveto the arc voltage of said commutation switch, said energy absorbingcircuit and said series circuit each being connected in parallel withsaid commutation switch, said capacitor being precharged so that uponopening of said commutation switch the discharge current from saidcapacitor'will flow into said commutation switch in a direction oppositethat of the current to be interrupted, said discharge current being ofsuch magnitude that the current to be interrupted will commutate to atleast said energy absorbing high resistance circuit wherein the currentis reduced to a quiescent current value which is switched off by acircuit breaker subsequentlyconnected in said 'main, the improvementwherein said circuit arrangement further includes circuit means forinfluencing the precharging of said capacitor and the capacitordischarge current path so that sufflcient time is available to saidcommutation switch during the commutation phase to increase its arcresistance to such an extent that the current flowing therein which isto be interrupted is commutated into the parallelly connected circuitsand cannot commutate back into said commutation switch;

wherein said circuit means includes a further commutation switchconnected in series with said first mentioned commutation switch and aseries circuit, including a further capacitor and a further auxiliaryswitch means responsive to the arc voltage across said furthercommutation switch, connected across said further commutation switch,said further capacitor having a large capacitance but a low ratedvoltage; wherein said first mentioned capacitor has a low capacitancebut a high rated voltage; and wherein said series circuit and saidenergy absorbing circuit are connected in parallel with the seriesconnection of said commutation switches, whereby a discharge currentflowing through said first mentioned capacitor after the arc has beenquenched in said further commutation switch effects a zero current flowthrough said first mentioned commutation switch causing commutation ofthe current to be interrupted to said energy absorbing high resistancecircuit.

l II

1. In a circuit arrangement for the current limiting interruption ofdirect and alternating currents at high volTages flowing in a power mainhaving a commutation switch connected in said main, an energy absorbinghigh resistance circuit, and a series circuit including a capacitor, aleakage inductance and a normally open auxiliary switch means responsiveto the arc voltage of said commutation switch, said energy absorbingcircuit and said series circuit each being connected in parallel withsaid commutation switch, said capacitor being precharged so that uponopening of said commutation switch the discharge current from saidcapacitor will flow into said commutation switch in a direction oppositethat of the current to be interrupted, said discharge current being ofsuch magnitude that the current to be interrupted will commutate to atleast said energy absorbing high resistance circuit wherein the currentis reduced to a quiescent current value which is switched off by acircuit breaker subsequently connected in said main, the improvementwherein said circuit arrangement further includes circuit means forinfluencing the precharging of said capacitor and the capacitordischarge current path so that sufficient time is available to saidcommutation switch during the commutation phase to increase its arcresistance to such an extent that the current flow therein which is tobe interrupted is commutated into the parallelly connected circuits andcannot commutate back into said commutation switch; and wherein saidcircuit means includes an additional inductance connected in series withsaid capacitor, said additional inductance being designed so that itdelays the commutation process to the extent required for an increase inthe arc resistance of said commutation switch, and means for causingsaid capacitor to be charged only during the commutation process.
 2. Thecircuit arrangement defined in claim 1 wherein said circuit meansincludes means for charging said capacitor by the arc voltage.
 3. Thecircuit arrangement defined in claim 2 wherein said means for chargingcauses said capacitor to be charged to a value dependent on theintensity of the current to be interrupted.
 4. The circuit arrangementdefined in claim 3 wherein said means for charging said capacitorcomprises: a measuring resistor connected in series with saidcommutation switch and an amplifier circuit having its signal inputconnected across said measuring resistor and its supply voltage inputconnected across said commutation switch said amplifier provided theoutput value iq f (i, Vb) where iq is the charging current for saidcapacitor, i is the current to be interrupted and Vb is the arc voltageacross said commutation switch.
 5. The circuit arrangement defined inclaim 3 wherein said auxiliary switch means comprises a pair of sparkgaps, one of which is connected on each side of said capacitor, in saidseries circuit, and wherein said charging means comprises a pair ofresistive circuit paths connecting the terminals of said capacitoracross said commutation switch.
 6. The circuit arrangement defined inclaim 5 wherein said capacitor includes a plurality of capacitorsconnected in series; wherein said pair of resistive circuit pathsconnects the terminals of each of said capacitors in parallel acrosssaid commutation switch so as to charge each capacitor in the samepolarity; and wherein an additional spark gap is connected in saidseries circuit between each pair of capacitors, whereby a multipliertype charging circuit is provided.
 7. The circuit arrangement defined inclaim 6 wherein there are n capacitors and wherein all n chargingcurrents of the multiplier circuit have the same time constant and thecapacitors are all charged, after commutation of the current to beinterrupted into the parallely connected high resistance circuit branchwith identical voltages.
 8. The circuit arrangement defined in claim 6wherein the charge time constants of said charging means are dimensionedso that the capacitors are sufficiently charged in a few milLisecondsbefore the commutation of the current to be interrupted occurs andduring the duration of the arc formed by the opening of said commutationswitch.
 9. The circuit arrangement defined in claim 6 wherein said sparkgaps are of sufficient size so that they can carry a current of fromseveral hundred to one thousand amperes for several tens ofmilliseconds, and wherein the charging resistances in said resistivepaths are dimensioned so that they simultaneously serve as energyabsorbers.
 10. In a circuit arrangement for the current limitinginterruption of direct and alternating currents at high voltages flowingin a power main having a commutation switch connected in said main, anenergy absorbing high resistance circuit, and a series circuit includinga capacitor, a leakage inductance and a normally open auxiliary switchmeans responsive to the arc voltage of said commutation switch, saidenergy absorbing circuit and said series circuit each being connected inparallel with said commutation switch, said capacitor being prechargedso that upon opening of said commutation switch the discharge currentfrom said capacitor will flow into said commutation switch in adirection opposite that of the current to be interrupted, said dischargecurrent being of such magnitude that the current to be interrupted willcommutate to at least said energy absorbing high resistance circuitwherein the current is reduced to a quiescent current value which isswitched off by a circuit breaker subsequently connected in said main,the improvement wherein said circuit arrangement further includescircuit means for influencing the precharging of said capacitor and thecapacitor discharge current path so that sufficient time is available tosaid commutation switch during the commutation phase to increase its arcresistance to such an extent that the current flowing therein which isto be interrupted is commutated into the parallelly connected circuitsand cannot commutate back into said commutation switch; wherein saidcircuit means includes a further commutation switch connected in serieswith said first mentioned commutation switch and a series circuit,including a further capacitor and a further auxiliary switch meansresponsive to the arc voltage across said further commutation switch,connected across said further commutation switch, said further capacitorhaving a large capacitance but a low rated voltage; wherein said firstmentioned capacitor has a low capacitance but a high rated voltage; andwherein said series circuit and said energy absorbing circuit areconnected in parallel with the series connection of said commutationswitches, whereby a discharge current flowing through said firstmentioned capacitor after the arc has been quenched in said furthercommutation switch effects a zero current flow through said firstmentioned commutation switch causing commutation of the current to beinterrupted to said energy absorbing high resistance circuit.